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Abstract
The detection of cosmic neutrinos with energies above a teraelectronvolt (TeV) offers a unique exploration into astrophysical phenomena$^{1, 2–3}$. Electrically neutral and interacting only by means of the weak interaction, neutrinos are not deflected by magnetic fields and are rarely absorbed by interstellar matter: their direction indicates that their cosmic origin might be from the farthest reaches of the Universe. High-energy neutrinos can be produced when ultra-relativistic cosmic-ray protons or nuclei interact with other matter or photons, and their observation could be a signature of these processes. Here we report an exceptionally high-energy event observed by KM3NeT, the deep-sea neutrino telescope in the Mediterranean Sea$^{4}$, which we associate with a cosmic neutrino detection. We detect a muon with an estimated energy of $12{0}_{-60}^{+110}$ petaelectronvolts (PeV). In light of its enormous energy and near-horizontal direction, the muon most probably originated from the interaction of a neutrino of even higher energy in the vicinity of the detector. The cosmic neutrino energy spectrum measured up to now$^{5, 6–7}$ falls steeply with energy. However, the energy of this event is much larger than that of any neutrino detected so far. This suggests that the neutrino may have originated in a different cosmic accelerator than the lower-energy neutrinos, or this may be the first detection of a cosmogenic neutrino$^{8}$, resulting from the interactions of ultra-high-energy cosmic rays with background photons in the Universe.
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de Wasseige, G., Genton, E., Lamoureux, M., Lazar, J., Lemaitre, V., Mauro, J., Myhr, P., KM3NeT, & et al. (2025). Observation of an ultra-high-energy cosmic neutrino with KM3NeT. Nature Physics, 638, 376. https://doi.org/10.1038/s41586-024-08543-1 (Original work published 2025)